In U.S. Pat. No. 3,351,697, there is described an apparatus for longitudinally stretching thermoplastics material films in which the stretching zone comprises a plurality of rolls. Such an apparatus is capable of stretching the film to up to 2.5 times its original length.
To prevent the width of the film from contracting during the longitudinally stretching, rolls are used which have diameters of less than 40 mm. This is because it has hitherto been assumed that if the film is looped around at least 50% of each individual roll, the film will be sufficiently supported to prevent the width from contracting. If the rate of stretching does not exceed a pre-determined value, usually of the order of 10 m/min, satistactory results can be achieved utilising such an apparatus.
However, there are certain disadvantages associated with the manufacture of stretched films using such an apparatus. Firtsly, although the rolls have a maximum diameter of 40 mm, they may have a length of up to 2600 mm. The rolls therefore tend to sag under their own weight. Such sagging is highly undesirable and it is consequently necessary to ensure that the spacings between adjacent individual rolls are small. More importantly, considerable problems arise with regard to the tempering and drive of such small diameter rolls.
In British Patent Specification No. 1,394,805, there is described a thermoplastics material stretching apparatus in which the stretching zone is formed from individual rolls. To prevent the width of the film from contracting whilst it is being longitudinally stretched, the adhesion of the film to the rolls is enhanced by electrostatically charging the film.
Both of these apparatuses have the additional disadvantage that they only operate at very low speeds. If the plastics material film has been electrostatically charged, it is very difficult to remove the charge therefrom. It is therefore very difficult to prevent particles of dust from being attracted to the charged film during further processing. Moreover, it is also difficult to remove the film from such a roll in a tension-free manner.
Hitherto unknown problems also arise if an attempt is made to effect longitudinally stretching of film at high speeds. In particular, it is not possible, using these known apparatuses, to prevent the width of the film web from contracting. This is because, at high speed, the film web does not adhere well to the individual stretching rolls. Air enters the gap between the film web and the external surface of the stretching rolls. This causes a cushion of air to be formed in the region in which the film loops around the stretching rolls. This air cushion permits the release of plasticiser residues and other liquid components from the film, which residues settle on the roll surface. This, in turn, causes specks to be formed on the film.